| AES is an iterated block cipher with a variable block length and a variable key length. There are 3.4 x 10 possible 128-bit AES keys; in comparison, DES keys are 56 bits long, which means there are approximately 7.2 x 10 possible DES keys. Thus, there are of the order of 10 times more AES 128-bit keys than DES 56-bit keys. In the late 1990s, specialized "DES Cracker" machines were built that could recover a DES key within a few hours. In other words, by trying possible key values, the hardware could determine which key was used to encrypt a message. The chances that someone could use the "DES Cracker" like hardware to crack an AES key are close to zero. Assuming that one could build a machine that could recover a DES key in a second (i.e., try 2 keys per second), then it would take that machine approximately 149 thousand-billion (149 trillion) years to crack a 128-bit AES key. To put that into perspective, the universe is believed to be less than 20 billion years old.
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